Electrochemical fuel cells generate electrical energy by converting chemical energy derived from a fuel directly into electrical energy by the oxidation of the fuel in the cell. A typical fuel cell includes an anode, a cathode and an electrolyte. Fuel and oxidant are supplied to the anode and cathode, respectively. At the anode, the fuel permeates the electrode material and reacts at the catalyst layer to form cations, which migrate through the electrolyte to the cathode. At the cathode, the oxygen-containing gas supply reacts at the catalyst layer to form anions. The anions formed at the cathode react with the cations to form a reaction product. The fuel cell generates a usable electric current and the reaction product is removed from the cell.
In electrochemical fuel cells employing hydrogen as the fuel and oxygen-containing air (or substantially pure oxygen) as the oxidant, a catalyzed reaction at the anode produces hydrogen cations from the fuel supply. The ion exchange membrane facilitates the migration of hydrogen ions (protons) from the anode to the cathode. In addition to conducting hydrogen cations, the membrane isolates the hydrogen fuel stream from the oxidant stream, typically comprising oxygen-containing air. At the cathode, oxygen reacts at the catalyst layer to form anions. The anions formed at the cathode react with the hydrogen ions that have crossed the membrane to form liquid water as the reaction product.
The anode and cathode reactions in such fuel cells is shown in equations (1) and (2) below: EQU Anode reaction H.sub.2 .fwdarw.2H.sup.+ +2e.sup.- ( 1) EQU Cathode reaction 1/20.sub.2 +2H.sup.30 2 +2e.sup.- .fwdarw.H.sub.2 O (2)
Solid polymer fuel cells generally contain a membrane electrode assembly ("MEA") consisting of a solid polymer electrolyte or ion exchange membrane disposed between two electrodes formed of porous, electrically conductive sheet material. The electrodes are typically formed of carbon fiber paper, and are usually impregnated or coated with a hydrophobic polymer, such as polytetrafluoroethylene. The MEA contains a layer of catalyst at each membrane/electrode interface to induce the desired electrochemical reaction. A finely divided platinum catalyst is typically employed.
In conventional fuel cells, the MEA is disposed between two rigid, electrically conductive separator plates, each of which has at least one flow passage or groove engraved, milled or molded in the surface facing the MEA. These separator plates, sometimes referred to as flow field plates, are typically formed of graphite. The flow passages in the separator plates direct the fuel and oxidant to the respective electrodes, namely, the anode on the fuel side and the cathode on the oxidant side. The separator plates are electrically coupled in series to provide a path for conducting electrons between the electrodes.
In conventional single cell arrangements, separator plates are located on both the anode and cathode sides of each individual fuel cell. The separator plates intimately contact the respective electrodes to provide a conductive path to carry electrons formed at the anode to the cathode to complete the electrochemical reaction. The separator plates thus perform several functions: (1) they act as current collectors, (2) they provide mechanical support for the electrodes, (3) they provide access channels for the fuel and oxidant to the respective anode and cathode surfaces, and (4) they provide channels for the removal of water formed during operation of the cell.
Two or more individual fuel cells can be connected together in series or in parallel to increase the overall power output of the assembly. In such arrangements, the cells are typically connected in series, wherein one side of a given separator plate contacts the anode of one cell and the other side of the separator plate contacts the cathode of the adjacent cell. Such a series connected multiple fuel cell arrangement is referred to as a fuel cell stack, and is usually held together by tie rods and end plates. The stack typically includes feed manifolds or inlets for directing fuel (substantially pure hydrogen, methanol reformate or natural gas reformate) to the flow field passages of the separator plate on the anode side of each fuel cell and oxidant (substantially pure oxygen or oxygen-containing air) to the flow field passages of the separator plate on the cathode side of each fuel cell. The stack also usually includes a feed manifold or inlet for directing the coolant fluid, typically water, to interior channels within the stack to absorb heat generated by the exothermic reaction of hydrogen and oxygen within the fuel cells. The stack also generally includes exhaust manifolds or outlets for expelling unreacted fuel carrying entrained water and for expelling unreacted oxidant carrying entrained water, as well as an outlet manifold for the coolant water exiting the stack.
Perfluorosulfonic ion exchange membranes, such as those sold by DuPont under its Nafion.RTM. trade designation, have been found effective for use in electrochemical fuel cells. Nafion.RTM. membranes must be hydrated with water molecules for ion transport to occur. Such hydration typically occurs by humidifying the fuel and oxidant streams prior to introducing the streams into the cell.
A new type of experimental perfluorosulfonic ion exchange membrane, sold by Dow under the trade designation XUS 13204.10, has also been found effective for use in electrochemical fuel cells. Like Nafion.RTM. membranes, the Dow experimental membranes appear to require some hydration to effect hydrogen ion transport.
In fuel cells employing hydrogen as the fuel and oxygen as the oxidant, the fuel can be supplied in the form of substantially pure hydrogen or as a hydrogen-containing reformate as, for example, the product of the reformation of methanol and water or reformation of natural gas. Similarly, the oxidant can be supplied in the form of substantially pure oxygen or oxygen-containing air. The fuel cells are typically flooded with fuel and oxidant at constant pressure. Pressure is generally controlled by pressure regulators at the source of the fuel and oxidant reactant streams. When an electrical load is placed on the circuit bridging the electrodes, fuel and oxidant are consumed in direct proportion to the electrical current drawn by the load.
As previously stated, a fuel cell stack usually includes a feed manifold or inlet for directing the coolant fluid, typically water, to interior channels within the stack to absorb heat generated by the exothermic reaction of hydrogen and oxygen within the fuel cells. In conventional designs, the interior coolant channels are generally formed by the cooperating surfaces of two separator plates, one of which contains grooves engraved, milled or molded in its surface and the other of which is planar. The coolant channels are located at periodic intervals along the stack. In the present invention, the separator plates are too thin to have coolant grooves formed in their surfaces. Consequently, the fuel cell stacks of the present invention employ separate cooling jackets or layers having grooves formed therein which cooperate with the planar surface of an adjacent separator plate to form the coolant channels. Alternatively, the membrane electrode assembly itself could be formed with coolant channels or capillaries, such as, for example, tubes running through the electrode sheet material for carrying the coolant through the assembly, thereby eliminating the need for a separate coolant jacket. In addition, cooling could be accomplished by the passage of a coolant fluid, such as air, over heat transfer surfaces such as fins projecting from the separator plates.
The "repeating unit" of a fuel cell stack is the smallest recurring portion of the stack which includes at least one membrane electrode assembly, as well as the separator plates and the cooling jacket(s) associated with the membrane electrode assembly. Since one cooling jacket may not be present for each membrane electrode assembly (i.e., a cooling jacket could in some instances provide cooling to multiple membrane electrode assemblies), a repeating unit may include more than one membrane electrode assembly.
Conventional repeating units, which include membrane electrode assemblies interposed between two rigid separator plates generally formed of graphite, are disadvantageous in several respects. First, the separator plates, must be formed thick enough to accommodate engraved, milled or molded flow passages. The thickness of the separator plates increases the weight and volume of the fuel cell. Because the reactant flow passages in the present invention are in the electrodes themselves, the separator plates need not accommodate the flow passages, and the separator plates may be formed of thinner material than in conventional assemblies. In the present invention, the separator plates are formed of a thin sheet of electrically conductive material. In one embodiment of the present invention, the electrodes have a flow passage engraved, milled or molded in the surface facing the adjacent separator plate. The electrode surface cooperates with the surface of the separator plate to complete the reactant flow passage. A thin, lightweight, electrically conductive sheet material, such as graphite foil or suitable metal like niobium or titanium for example, may be used as a separator plate. The reduction in the weight and volume of the separator plates in the present invention permits the manufacture of lighter, more compact fuel cells stacks, and provides a higher fuel cell power-to-weight ratio and a higher power-to-volume ratio than in conventional fuel cells. Furthermore, because the separator plates can be made of thinner material, the separator plates in the present invention are less expensive to manufacture than the separator plates in conventional fuel cells.
An additional benefit of the present design is the proximity of the reactant (fuel and oxidant) flow to the catalyst sites. Because the reactants flow through passages in the electrode material itself, as opposed to channels in the separator plate adjacent the electrode, the reactants need not migrate through the entire thickness of the electrode material to reach the catalyst layers. Decreasing the distance through which the reactants must travel to reach the catalyst layer enhances the access of the reactants to the catalyst layer and improves potential fuel cell performance.
Another advantage of the present invention over conventional fuel cell designs is the relative ease of forming flow passages in the electrode material as opposed to engraving, milling or molding passages in the rigid separator plates. Electrodes are generally composed of porous electrically conductive sheet material such as carbon fiber paper, while the rigid separator plates are generally formed of graphite or suitable metal. In the present invention, rigid separator plates need not be engraved, milled or molded, but can be replaced by thin, lightweight sheets of nonporous electrically conductive material, which can be processed using less time and expense than in the milling of graphite or metal separator plates.